Method for manufacturing foamable injection molding and injection device therefor

ABSTRACT

A plasticizing process in which a molten resin that has been plasticized and dissolved in a foaming gas is sent to the front end side of a heating cylinder (20) by means of a screw (21) that rotates in a heating cylinder (20), and the screw (21) retreats according as the molten resin accumulates in the front end side of the heating cylinder (20); a transfer process in which the screw (21) is advanced with rotating and transfers the molten resin accumulated in the front end side of the heating cylinder (20) from a plasticizing unit (2) to an injection unit (3), and an injection process in which the molten resin is injected from the injection unit (3) are conducted and repeated to produce a foamable injection molded product.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a National Stage of International Application No.PCT/JP2014/000876 filed Feb. 20, 2014, claiming priority based onJapanese Patent Application No. 2013-036044 filed Feb. 26, 2013, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a method for producing foamableinjection molded product such as a preform that is obtained bydissolving a foaming gas such as an inert gas a thermoplastic resin in athermoplastic resin, followed by injection molding, as well as to aninjection apparatus for producing such foamable injection moldedproduct.

BACKGROUND ART

Conventionally, a synthetic resin bottle obtained by a process in whicha preform with a bottomed cylindrical shape (an injection molding) isformed by injection molding by using a thermoplastic resin such aspolyethylene terephthalate, and this preform is molded by biaxialstretch blow molding or the like has been generally used in a wide rangeof fields as a container that accommodates contents such as variousbeverages.

In such a synthetic resin bottle, in order to allow contents that areeasily changed in quality by light to be accommodated, known is atechnology of imparting light-shielding properties by compounding acolorant such as a pigment with the thermoplastic resin.

However, in recent years, used bottles are collected and recycled as arecycled resin in various applications. Under such circumstances, thereis a problem that application of recycled resins is restricted in thecase where a colorant is mixed into the resins. Therefore, as atechnology of imparting light-shielding properties by distributing foamson the bottle wall without compounding a colorant, proposed is atechnology in which a foaming gas such as an inert gas is dissolved in athermoplastic resin to allow fine cells to be generated when a preformis produced by injection molding, and then, this preform is foamed by aheating and blowing process in biaxial stretch blow molding, whereby thepreform is molded into a prescribed bottle shape.

On the other hand, one of the applicants of the present applicationpreviously proposed a technology of further improving light-shieldingproperties and suppressing deterioration in appearance that is caused bypresence of foams (see Patent Document 1).

In such a technology, a foaming gas such as an inert gas is dissolved ina resin and generation of foams is suppressed at the time of injectionmolding of a preform to obtain a preform in a non-foamed state, and thispreform is foamed by a heating and blowing process in biaxial stretchblow molding, whereby the preform is molded into a bottle shape that hasa large amount of small cells being distributed therein.

On the other hand, as a method for injection molding the above-mentionedfoamable injection molded product, a technology utilizing a 2-stagepre-plasticizing injection apparatus that comprises a plasticizing unitand an injection unit has been proposed (see Patent Document 2). In thistechnology, a molten resin in which a foaming gas such as an inert gasis dissolved in the plasticizing unit is injected into an injection moldby means of a reciprocally-moving plunger that is provided in aninjection unit. This apparatus is provided with an accumulator in whichthe molten resin is accumulated through a conduit prior to theinjection.

RELATED ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A-2008-94495-   Patent Document 2: Japan Patent No. 4460074

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

When a synthetic resin-made foamed bottle disclosed in Patent Document 1or the like is mass-produced, in production of an injection moldedproduct such as a preform that is obtained by using a large amount of aresin with a short molding cycle, use of a 2-stage pre-plasticizinginjection apparatus disclosed in Patent Document 2 mentioned above ispossible.

However, in injection molding of a foamable injection molded productsuch as a preform in which a foaming gas such as an inert gas isdissolved is subjected to injection molding, it is required to allow afoaming gas to be dissolved homogenously in a molten resin during aplasticizing process (i.e. a resin-melting process). Therefore, theresin pressure of a molten resin is required to be set higher than thatin normal injection molding, and the plasticizing time is required to beprolonged. Accordingly, it is desired that the screw rotation of anextrusion unit and gas injection time during the molding cycle beincreased in order to improve plasticization capacity and gas dissolved,thereby to improve productivity.

Under such circumstances, in 2-stage pre-plasticizing injection moldingof a foamable injection molded product in which a foaming gas such as aninert gas is dissolved in an injection molded product such as a preform,the inventors have made intensive studies in respect of increasing theplasticizing time of a raw material resin and a gas injection timeduring the molding cycle, thereby to improve plasticization capacity andgas dissolving. As a result, the inventors have completed the presentinvention.

That is, an object of the present invention is to provide a method forproducing a foamable injection molding that is capable of, in injectionmolding of a foamable injection molded product such as a preform,increasing the time of plasticizing a raw material resin and prolongingthe time of injecting a foaming gas to a plasticized molten resin,thereby improving plasticizing of a raw material resin and impregnationof a foaming gas in a molten resin, as well as to an injection apparatusfor producing such a foamable injection molded product.

Means for Solving the Problems

The method for producing a foamable injection molded product accordingto the present invention is a method using an injection apparatuscomprising a plasticizing unit that plasticizes a raw material resin andan injection unit that injects a plasticized molten resin, and theplasticizing unit is provided with a heating cylinder having a foaminggas injection valve and a screw provided inside the heating cylinder,

the method comprises:

a plasticizing process wherein the raw material resin is plasticized bymeans of the screw provided inside the heating cylinder of theplasticizing unit, the foaming gas injected from the foaming gasinjection valve is dissolved in the plasticized molten resin, and thescrew retreats according as the molten resin accumulates in the frontend side of the heating cylinder by the screw that is rotating, and

a transfer process wherein the screw that has been retracted to apredetermined position is caused to advance while rotating, and themolten resin which has been accumulated in the front end side of theheating cylinder is transferred to the injection unit,

wherein, when the transfer process is completed, the plasticizingprocess re-starts without stopping the rotation of the screw and aninjection process in which the molten resin is injected from theinjection unit is conducted, and

when the injection process is completed, the plasticizing process iscompleted and the transfer process re-starts.

Further, the injection apparatus according to the present inventioncomprises a plasticizing unit that plasticizes a raw material resin andan injection unit that injects a plasticized molten resin, wherein theplasticizing unit is provided with a heating cylinder having a foaminggas injection valve and a screw provided inside the heating cylinder,and

the rotation and the advance and retreat movements of the screw arecontrolled such that

a raw material resin is plasticized by means of the screw inside theheating cylinder of the plasticizing unit and a foaming gas injectedfrom a foaming gas injection valve is dissolved in the plasticizedmolten resin, and the screw retreats according as the molten resinaccumulates in the front end side of the heating cylinder by the screwthat is rotating,

the screw advances with rotating to cause the molten resin which hasbeen accumulated in the front end side of the heating cylinder to betransferred to the injection unit.

Advantageous Effects of the Invention

According to the present invention, in producing a foamable injectionmolded product by injecting molding a thermoplastic resin in which afoaming gas is dissolved, the time of plasticizing a raw material resinand the time of injecting a foaming gas to a plasticized molten resin inthe molding cycle are increased, plasticizing performance andimpregnation of a foaming gas are improved, leading to an increase inproductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing an outline of an injectionapparatus according to an embodiment of the invention;

FIG. 2 is an explanatory view showing one example of a rotary valve inan injection e apparatus according to an embodiment of the invention;

FIG. 3 is an explanatory view showing one example of a rotary valve inan injection apparatus according to an embodiment of the invention;

FIG. 4 is a process drawing showing an outline of a method for producingfoamable injection molded product according to an embodiment of theinvention; and

FIG. 5 is a correlation diagram showing one example of the relationshipbetween the injection time of a foaming gas, the position of a screw,the rotational speed of a screw and the resin pressure of a molten resinaccumulated on the front end side of a heating cylinder of aplasticizing unit.

MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, an explanation will be made on a preferred embodiment ofthe invention with reference to the drawings.

[Injection Apparatus]

An injection apparatus 1 shown in FIG. 1 has a plasticizing unit 2 forplasticizing a raw material resin that has been fed and an injectionunit 3 for injecting a plasticized molten resin.

In the injection apparatus 1 shown in FIG. 1, a heating cylinder 20 thatis provided in the plasticizing unit 2 and a heat-retaining cylinder 30that is provided in the injection unit 3 are arranged in parallel toeach other, and the front end sides of these cylinders are connectedthrough a connection part 4 having a rotary valve 40 in its inside. Tothe front end side of the heat-retaining cylinder 30 provided in theinjection unit 3, a nozzle 5 to be attached to a mold (not shown) isconnected through the connection part 4.

As shown in FIG. 2 and FIG. 3, the rotary valve 40 is provided with across head 41 that has a passage with a T-shaped cross section and ahollow part that has a circular cross section and is formed around theintersection of the passage, and a valve 42 that is rotatably attachedto the inside of the hollow part formed in the cross head 41 and has apassage having a T-shaped cross section.

Further, as shown in FIG. 2, by switching the rotatory valve 40 to closethe nozzle part 5 and to connect the plasticizing unit 2 and theinjection unit 3, a molten resin that has been plasticized in theplasticizing unit 2 can be transferred to the injection unit 3. On theother hand, as shown in FIG. 3, by switching the rotary valve 40 toclose the plasticizing unit 2 and to connect the injection unit 3 andthe nozzle part 5, a molten resin that has been transferred to theinjection unit 3 can be injected from the nozzle part 5.

Within the heating cylinder 20 that is provided in the plasticizing unit2, a screw 21 having a helical wing part 22 is arranged. By a screwdriving part, a screw advancing mechanism and a back pressure controlpart (all are not shown), the rotation, the advance and retreatmovements of the screw 21 in the heating cylinder 20 and the resinpressure at the time of plasticizing can be controlled.

On the rear end side of the heating cylinder 20, a hopper 25 for feedinga pelletized raw material resin to the heating cylinder 20 is provided.

The raw material resin that has been fed to the heating cylinder 20 fromthe hopper 25 is sent to the front end side of the screw 21 by the screw21 that rotates within the heating cylinder 20. At the same time, whilebeing sheared, the raw material resin is molten by shear heat and heatof the heater provided in the heating cylinder 20, and is plasticized.The raw material resin that is plasticized in this way is sent to thefront end side of the heating cylinder 20 while being kneaded by therotation of the screw 21.

Further, in the heating cylinder 20, foaming gas injection valves 24 aand 24 b for injecting a foaming gas which is to be dissolved in theplasticized molten resin are provided. As the foaming gas, an inert gassuch as a carbon dioxide gas or a nitrogen gas can be used.

Also, a check valve 23, that opens in a direction in which the moltenresin that has been fed from the hopper 25 and plasticized is sent tothe front end side of the heating cylinder 20 and does not open in theopposite direction, is provided at the screw 21. Then, a foaming gas isdissolved in the plasticized molten resin that has been sent to thefront end side of the heating cylinder 20 by the rotation of the screw21 and has passed the check valve 23.

Then, by the check valve 23 provided at the screw 21, flowing out of thefoaming gas that has been injected to the heating cylinder 20 to therear end side of the heating cylinder 20 is prevented, and as a result,lowering in resin pressure of the molten resin to be sent to the frontend side of the heating cylinder 20 is avoided, whereby foaming of afoaming gas which is dissolved in the molten resin caused by reductionin pressure in the heating cylinder 20 is suppressed.

Further, the advance and retreat movements of the screw 21 arecontrolled such that according as the molten resin that has beenplasticized and in which a foaming gas has been dissolved accumulates inthe front end side of the heating cylinder 20, the screw 21 retreats insuch a manner that it is pushed back by the resin pressure, and then thescrew 21 advances when a prescribed amount of the plasticized moltenresin is accumulated in the front end side of the heating cylinder 20.At this time, by keeping the resin pressure of the molten resin at aconstant level or at a level that is equal to or higher than theconstant level by controlling the retreating of the screw 21 by a backpressure control part of the plasticizing unit 2, foaming of theimpregnating inert gas can be suppressed.

As mentioned above, the plasticizing unit 2 transfers the plasticizedmolten resin in which a foaming gas to the injection unit 3 by advancingthe screw 21 that has retreated to a prescribed position due toaccumulation in the front end side of the heating cylinder 20 of themolten resin in which a foaming gas is dissolved. At this time, thestroke length for which the screw 21 advances and retreats can beappropriately set in accordance with the amount of the molten resin tobe sent to the injection unit 3, i.e. the dimension of an injectionmolding such as a preform and the number of cavities in a mold.

The position of a foaming gas injection valve 24 provided on the heatingcylinder 20 can be set in accordance with the stroke length for whichthe screw 21 advances and retreats such that, even when the screw 21reaches the position where the screw 21 most retreats (hereinafterreferred to as the “retreat limit”), the foaming gas injection valve 24is positioned rearer than the front end of the screw 21. The number ofthe foaming gas injection valve 24 provided on the heating cylinder 20may be one. In the example shown in FIG. 1, plural (two) foaming gasinjection valves 24 a and 24 b are provided along the direction of theadvance and retreat of the screw 21. When plural foaming gas injectionvalves 24 are provided, in accordance with the position of the screw 21that advances and retreats within the heating cylinder 20, the foaminggas injection valve 24 is switched such that a foaming gas is injectedfrom the foaming gas injection valve 24 that is positioned rearer thanthe front end of the screw 21.

For example, in the examples shown in FIG. 4(a) and FIG. 4(b), in theprocess during which the screw 21 retreats, among the two foaming gasinjection valves 24 a and 24 b, the foaming gas injection valve 24 aprovided nearer to the front end side of the heating cylinder 20 ispositioned on the front end side relative to the front end side of thescrew 21. In such a case, injection of a foaming gas from the foaminggas injection valve 24 a may be stopped, and the foaming gas injectionvalve 24 may be switched such that a foaming gas is injected from thefoaming gas injection valve 24 b that is positioned on the rear end siderelative to the front end of the screw 21.

The position of the check valve 23 provided on the screw 21 can be setsuch that, even when the screw 21 reaches the position where the screw21 most advances (hereinafter referred to as the “advance limit”), thecheck valve 23 can be positioned on the rear end side of the heatingcylinder 20 on which the foaming gas injection valve 24 is provided.When plural check valves 23 are provided, the valves may be positionedon the front end side of the heating cylinder 20 on which the foaminggas injection valve 24 is provided. In such a case, the foaming gasinjection valve 24 may be switched such that injection of an inert gasfrom the foaming gas injection valve 24 stops.

In this embodiment, the check valve 23 is provided on the screw 21. Thischeck valve 23 is provided primarily with an aim of preventing themolten resin in which a foaming gas that has been injected to theheating cylinder 20 is dissolved from flowing out to the rear end sideof the heating cylinder 20. In this embodiment, a transfer process isconducted by allowing the screw 21 to advance while rotating. During theperiod of time for which the transfer process is conducted by the screw21, an internal pressure (thrust Pm) of the molten resin to be sent tothe front end side of the heating cylinder 20 is generated by therotation of the screw. Therefore, by designing and setting theconditions of the screw 21 such that the thrust Pm becomes larger thanthe transfer pressure (Pt), there will be no need to provide, on thefront end of the screw 21, a check valve or the like that preventsback-rush of the raw material resin.

As mentioned above, on the front end of the screw 21 of the presentinvention, no check valve is provided. Therefore, the helical wing part22 that plasticizes the raw material resin can be formed long enough toreach the front end of the screw 21. Accordingly, plasticizing of theraw material resin and kneading of an inert foaming gas and a moltenresin can be improved.

Inside the heat-retaining cylinder 30 provided in the injection unit 3,a plunger rod 31 as an injection member is arranged. By a plungerdriving part (not shown), the advance and retreat movements of theplunger 32 in the heat-retaining cylinder 30 are controlled.

In the plasticizing unit 2, the raw material resin is plasticized, andthe plasticized molten resin in which a foaming gas is dissolved. Such amolten resin is then transferred to the injection unit 3, and isaccumulated in the heat-retaining cylinder 30 of the injection unit 3.According as the molten resin accumulates in the heat-retaining cylinder30, the plunger rod 31 is pushed back and retreats by the resin pressureof the molten resin accumulated in the heat-retaining cylinder 30. Atthis time, by appropriately adjusting the stroke length for which theplunger 32 advances and retreats, the molten resin to be injected isweighed. In order to prevent foaming of a foaming gas which is dissolvedin the molten resin, the plunger 32 retreats while keeping the resinpressure of the molten resin accumulated in the heat-retaining cylinder30 to be a constant level or be at a level that is equal to or higherthan the constant level. On the other hand, as for the plunger 32, theadvance and retreat movements thereof are controlled such that itadvances when a prescribed amount of the molten resin is accumulated inthe heat-retaining cylinder 30.

As mentioned above, in the injection unit 3, the plunger 32 retreats andadvances, whereby the molten resin accumulated in the heat-retainingcylinder 30 is weighed, and then a prescribed amount of the molten resinis injected from a nozzle part 5.

In this embodiment, by passing through a plasticizing process, atransfer process and an injection process explained below using theabove-mentioned injection apparatus 1, a foamable injection moldedproduct is produced.

[Plasticizing Process]

First, in the plasticizing unit 2, a pelletized raw material resin thathas been fed from the hopper 25 to the heating cylinder 20 isplasticized.

In this embodiment, by injecting a foaming gas from the foaming gasinjection valve 24, a foaming gas is dissolved in the plasticized moltenresin. The timing of injecting a foaming gas will be stated later.

In this plasticizing process, the rotary valve 40 is switched as shownin FIG. 3, and a raw material resin that has been fed from the hopper 25is plasticized as mentioned above. As shown in FIG. 4(a) to FIG. 4(c) insequence, the screw 21 that rotates in the heating cylinder 20plasticizes the raw material resin. The screw 21 is pushed back andretreats by the resin pressure of the molten resin accumulated in thefront end side of the heating cylinder 20 while sending the raw materialresin in which a foaming gas is dissolved to the front end side of theheating cylinder 20.

FIG. 4(a) shows the state where the rotating screw 21 starts to advancein the initial stage of the plasticizing process. As shown in FIG. 4(b)and FIG. 4(c) in sequence, as the molten resin is accumulated in thefront end side of the heating cylinder 20, the screw 21 retreats untilit reaches a predetermined stroke or until a predetermined period oftime lapses.

[Transfer Process]

In the transfer process that is conducted subsequent to the plasticizingprocess, the molten resin that is plasticized and in which a foaming gasis dissolved in the plasticizing unit 2 is transferred to the injectionunit 3. Prior to this transfer, the rotary valve 40 is switched as shownin FIG. 2. When a predetermined amount of a molten resin is accumulatedin the front end side of the screw 21 that has retreated in theplasticizing process, as shown in FIG. 4(d) to FIG. 4(e) in sequence,the screw 21 advances while rotating. After that, if necessary, thescrew 21 may continue to rotate at the advance limit. As a result, aprescribed amount of the molten resin that is plasticized and in which afoaming gas is dissolved is transferred to the injection unit 3.

FIG. 4(d) shows a state in which the screw 21 starts to advance whilerotating in the initial stage of the transfer process. As shown in FIG.4(e) and FIG. 4(f) in sequence, a molten resin accumulated in the frontend side of the heating cylinder 20 is transferred to the injection unit3.

At this time, the screw 21 (see FIG. 4(c)) that has retreated until itreaches a prescribed stroke or until a prescribed period of time islapsed in the plasticization process, after the completion of theplasticization process and before the shifting to transfer process,depending on the cycle, stops rotation, and keeps the resin pressure ofthe molten resin accumulated in the front end side of the heatingcylinder 20 at a constant level or at a level that is equal to or higherthan the constant level, and advances together with the re-start of therotation of the screw 21 after the lapse of a prescribed period of time.In the later stage of the plasticization process, the resin pressure ofthe resin accumulated in the front end side of the heating cylinder 20may be kept at a constant level or at a level that is equal to or higherthan the constant level, and when the screw 21 retreats until it reachesa prescribed stroke or until a prescribed period of time lapses, thetransfer process may start, thereby to allow the screw 21 to advancewithout stopping the rotation of the screw 21.

Then, after the screw 21 has retreated to a prescribed position with therotation thereof being controlled, the screw 21 advances to start thetransfer process. By doing so, the resin pressure of the molten resinaccumulated in the front end side of the heating cylinder 20 can be keptat a constant level during a period of time from the completion of theplasticization process to the shifting of the transfer process.

The molten resin that has been transferred to the injection unit 3 isaccumulated in the heat-retaining cylinder 30 while pushing the plungerrod 31 back to allow it to retreat by the resin pressure (see FIG. 4(e)and FIG. 4(f)), and is weighed in a prescribed amount in accordance withthe stroke length of the plunger rod 31. When the plunger 32 retreats toa prescribed position and the transfer process is completed (see FIG.4(f)), the rotary valve 40 is switched as shown in FIG. 3, whereafterthe above-mentioned the plasticization processes shown in FIG. 4(a) toFIG. 4(c) in sequence re-start.

As for the timing when the plasticization process re-starts by switchingthe rotary valve 40, it may be conducted simultaneously with the timewhen screw 21 reaches the advance limit or after the screw 21 rotatesfor a certain period of time at the advance limit or before the screw 21reaches the advance limit. When shifting from the transfer process tothe plasticizing process, in order to prevent lowering in pressure ofthe molten resin between the front end of the screw 21 and theconnection part, the screw 21 keeps on rotating.

As mentioned above, by allowing the screw 21 to keep on rotating, theplasticizing performance is improved, and the resin pressure of themolten resin in the screw 21 is stabilized.

[Injection Process]

When the rotary valve 40 is switched as shown in FIG. 3 after thecompletion of the transfer process, the injection process is conductedin parallel with the plasticization process that has re-started.

In the injection process, as shown in FIG. 4(a) to FIG. 4(c) insequence, due to the advancement of the plunger rod 31 of the injectionunit 3, a prescribed amount of the molten resin that has been weighed isinjected from the nozzle part 5, and by a mold not shown, a foamableinjection molded product having a prescribed shape is molded.

Here, FIG. 4(a) shows the state where the plunger 32 starts to advancein the initial stage of the injection process. As shown in FIG. 4(b) andFIG. 4(c) in sequence, a prescribed amount of the molten resin that hasbeen weighed in the heat-retaining cylinder 30 is injected from thenozzle 5.

When the injection process is completed, the plasticizing process iscompleted (see FIG. 4(c)) and the transfer process re-starts (see FIG.4(d)). These processes are repeated.

[Timing of Injecting Inert Gas]

In this embodiment, the processes mentioned above are repeated toproduce a foamable injection molded product. The timing when a foaminggas starts to be injected to the molten resin that has been kneaded andplasticized by means of the rotating screw 21 can be controlled asfollows.

(1) Injection of a foaming gas starts simultaneously with the start ofthe transfer process or after the lapse of a predetermined period oftime after the start of the transfer process.

(2) Injection of a foaming gas starts after the start of the transferprocess and the screw 21 that advances in the heating cylinder 20reaches a predetermined position.

(3) Injection of a foaming gas starts after the completion of thetransfer process and after the lapse of a prescribed period of timeafter the plasticization process starts.

(4) Injection of a foaming gas starts after the completion of thetransfer process and after the screw 21 that advances in the heatingcylinder 20 reaches a prescribed position after the start of theplasticization process.

As for the timing of starting injection of a foaming gas, the besttiming is appropriately selected from (1) to (4) mentioned above. Forexample, If the injection period of a foaming gas is prolonged, the besttiming of starting injection can be selected from (1) or (2) taking intoconsideration the relation with other various conditions. Further, ifthe injection period of a foaming gas is shortened, the time of startinginjection can be selected from (3) or (4).

Parameters such as the injection pressure, the injection amount or thelike of a foaming gas are appropriately set and controlled in advancetaking into consideration the optimization of the injection molding.

In the case where timing of starting injection of a foaming gas iscontrolled in accordance with (1) to (4) mentioned above, therelationship between the injection period, the position of the screw 21,the rotation speed of the screw 21 and the resin pressure of the moltenresin accumulated in the front end side of the heating cylinder 20,timing of starting injection of a foaming gas, is controlled inaccordance with (1) to (4) mentioned above is shown in FIG. 5.

One example of the above-mentioned timing of starting injection of afoaming gas to the plasticized molten resin; i.e. (1), (2), (3) and (4);is shown in FIG. 5. In FIG. 5, injection period of a foaming gas isshown as a double line and a portion with no double line indicates aperiod during which injection is stopped.

The amount of movement of the position of the screw 21 is indicated by asolid line, and the position where the screw 21 most retreats andplasticization is completed (retreat limit) is indicated as Lba and theposition where the screw 21 is most advanced (advance limit) isindicated as Lad. Further, a variation in resin pressure of the moltenresin accumulated in the front end side of the heating cylinder 20 isindicated by a long dashed short dashed line.

By controlling the time of starting injection of a foaming gas and theinjection period of a foaming gas in the above-mentioned way, a foaminggas can be injected to the molten resin in the state being kneaded bythe screw 21 that is rotating. When shifting from the transfer processto the plasticizing process, the screw 21 that has advanced to theadvance limit or a position that is immediately before the advance limitin the transfer process starts to retreat while rotating. Before thecompletion of the plasticizing process, injection of a foaming gasstops. In other words, during a period of time from the start ofinjection in (1) to (4) mentioned above to the completion of theplasticizing process, a necessary amount of a foaming gas is injected.

Accordingly, after injection of a foaming gas stops, kneading of themolten resin in which a foaming gas is dissolved by the screw 21 that isrotating continues, whereby impregnation of a foaming gas in the moltenresin is conducted homogenously. As a result, a necessary amount of agas can be injected to the plasticized molten resin without affectingadversely the plasticizing time of the raw material resin in the moldingcycle, whereby improvement of plasticizing performance and impregnationof a foaming gas can be realized, thus leading to improvement inproductivity.

By allowing the time of starting injecting a foaming gas to theplasticized molten resin to be (1) or (2) mentioned above, a foaming gascan be injected after the start of the transfer, i.e. immediately afterthe start of the rotation of the screw. Whereby, for the period of timefrom the start of the rotation of the screw to the termination of therotation of the screw, a longer gas injection time can be ensured. As aresult, an increase in amount of injected gas can be possible, whereby agas can be injected uniformly and evenly during the plasticizingprocess. On the other hand, by allowing the time of starting injectionof a foaming gas to be (3) or (4) mentioned above, after the transferprocess is completed, a gas is injected when the screw monotonicallyretreats, so the resin pressure is relatively stabilized, and hence theamount of an injected gas can be controlled easily with a high accuracy.

When a foaming gas is injected from the foaming gas injection valve 24to the heating cylinder 20, following pressures are appropriatelyadjusted such that the resin pressure (Pm) of the molten resin beforepassing the check valve 23 becomes larger than the sum of the injectionpressure (P2) of a foaming gas and the transfer pressure (P3) of themolten resin that is sent to the front end side of the heating cylinder20 after passing the check valve 23 (Pm>P2+P3). As a result, adisadvantage that, when the plasticized molten resin is sent to thefront end side of the heating cylinder 20 by means of the rotating screw21, the check valve 23 is closed and hence the molten resin cannot passthe check valve 23 can be avoided.

Then, in order to avoid the disadvantage mentioned above, it ispreferred that, by means of a resin pressure sensor S that is attachedon the same circumference as that on which the injection gas injectionvalve 24 is attached, the total of P2 and P3 (P2+P3) be detected, andwhen the P2+P3 becomes equal to or larger than the predetermined valueof the resin pressure, the injection pressure of a foaming gas P2 belowered or the operation of the injection apparatus 1 be controlled tostop it.

In the present invention, that was explained above with reference to theembodiments, the following processes (A) to (D) are repeated to producea foamable injection molded product:

(A) a plasticizing process wherein the raw material resin is plasticizedby means of the screw 21 provided inside the heating cylinder 20 of theplasticizing unit 2, the foaming gas injected from the foaming gasinjection valve 24 is dissolved in the plasticized molten resin, and thescrew 21 retreats according as the molten resin accumulates in the frontend side of the heating cylinder 20 by the screw 21 that is rotating;(B) a transfer process wherein the screw 21 that has retreated to apredetermined position is caused to advance while rotating, and themolten resin which has been accumulated in the front end side of theheating cylinder 20 is transferred to the injection unit 3;(C) when the transfer process is completed, the plasticizing processre-starts without stopping the rotation of the screw 21 and an injectionprocess in which the molten resin is injected from the injection unit 3is conducted;(D) when the injection molding process is completed, the plasticizingprocess is completed and the transfer process re-starts.

By repeating the processes A to D above, a foamable injection moldedproduct is produced.

By producing a foamable injection-molded product as mentioned above, inthe transfer process, in particular, when transferring the plasticizedmolten resin in which a foaming gas is dissolved and that is accumulatedin the front end side of the heating cylinder 20 to the injection unit3, it becomes possible to allow the screw 21 to advance while rotating,i.e., to continue the melting process of the raw material resin withoutstopping, whereby the molding cycle is prevented from beingdeteriorated. The raw material resin can be sufficiently plasticized andkneaded. Further, by appropriately adjusting the rotation speed and theadvancing speed of the screw 21, a back pressure is applied to themolten resin in which the foaming gas is dissolved, whereby foaming ofthe foaming gas can be suppressed. As a result, a foamable injectionmolded product in which plasticization of the raw material resin anddissolution of a foaming gas are improved can be produced.

Accordingly, when a synthetic resin-made foamed bottle mentioned abovein which foams are distributed on the bottle wall is mass-produced, bydissolving an inert gas such as a carbon dioxide gas or a nitrogen gasas a foaming gas in a thermoplastic resin such as polyethyleneterephthalate by applying the present invention, thereby to produce apreform by injection molding, plasticizing of raw material resins in themolding cycle and impregnation of a preform with a foaming gas can beimproved. By subjecting the preform to biaxial stretch blow molding, asynthetic resin-made foamed bottle in which fine cells are uniformlydistributed can be mass-produced in a high yield.

Hereinabove, the present invention was explained with reference topreferred embodiments. However, the present invention is not restrictedto the above-mentioned embodiments. For example, as mentioned above, thepresent invention can be applied to a technology of producing a foamableinjection molded product in which a foaming gas such as an inert gas isdissolved in a thermoplastic resin and fine foams are formed at the timeof injecting molding an injection molding such as a preform. It isneedless to say that various modifications are possible within the scopeof the present invention.

As an example of application of the present invention, a technology inwhich a foamable injection molded product is molded by biaxial stretchblow molding in order to mass-produce a synthetic resin-made foamedbottle in which foams are distributed on the bottle wall was given. Thepresent invention is not limited to such a technical field, and can beapplied to various technical fields as a technology of producing afoamable injection molded product in which a foaming gas such as aninert gas is dissolved.

INDUSTRIAL APPLICABILITY

As mentioned above, the present invention can be applied to varioustechnical fields as a technology of producing a foamable injectionmolded product in which a foaming gas such as an inert gas is dissolved.

EXPLANATION OF REFERENTIAL NUMERALS

-   1. Injection apparatus-   2. Plasticizing unit-   20. Heating cylinder-   21. Screw-   23. Check valve-   24. Foaming gas injection valve-   3. Injection unit

The invention claimed is:
 1. A method for producing foamable injectionmolded product using an injection apparatus comprising a plasticizingunit that plasticizes a raw material resin and an injection unit thatinjects a plasticized molten resin, and the plasticizing unit isprovided with a heating cylinder having a foaming gas injection valveand a screw arranged inside the heating cylinder, the method comprises:a plasticizing process wherein the raw material resin is plasticized bymeans of the screw provided inside the heating cylinder of theplasticizing unit, the foaming gas injected from the foaming gasinjection valve is dissolved in the plasticized molten resin, and thescrew retreats according as the molten resin accumulates in the frontend side of the heating cylinder by the screw that is rotating, and atransfer process wherein the screw that has retreated to a predeterminedposition is caused to advance while rotating, and the molten resin whichhas been accumulated in the front end side of the heating cylinder istransferred to the injection unit; wherein, when the transfer process iscompleted, the plasticizing process re-starts without stopping therotation of the screw and an injection process in which the molten resinis injected from the injection unit is conducted, and when the injectionprocess is completed, the plasticizing process is completed and thetransfer process re-starts.
 2. The method for producing a foamableinjection molded product according to claim 1, wherein, after allowingthe screw to advance while rotating, the screw is further rotated,whereby the molten resin is transferred to the injection unit.
 3. Themethod for producing a foamable injection molded product according toclaim 1, wherein the foaming gas is injected to the molten resin whichhas been plasticized by means of the screw that is rotating.
 4. Themethod for producing a foamable injection molded product according toclaim 1, wherein, simultaneously with the start of the transfer processor after the lapse of a predetermined period of time from the start ofthe transfer process, the injection of a foaming gas starts.
 5. Themethod for producing a foamable injection molded product according toclaim 1, wherein, after the start of the transfer process and after thescrew that advances in the heating cylinder reaches a predeterminedposition, the injection of a foaming gas starts.
 6. The method forproducing a foamable injection molded product according to claim 1,wherein, after the completion of the transfer process and after thelapse of a predetermined period of time from the start of theplasticizing process, the injection of a foaming gas starts.
 7. Themethod for producing a foamable injection molded product according toclaim 1, wherein, after the completion of the transfer process and afterthe screw that advances inside the heating cylinder from the start ofthe plasticization process reaches a predetermined position, theinjection of a foaming gas starts.
 8. The method for producing afoamable injection molded product according to claim 1, wherein theinjection of a foaming gas stops before the completion of theplasticization process.
 9. The method for producing a foamable injectionmolded product according to claim 1, wherein the screw is provided witha check valve, and the molten resin which has been sent to the front endside of the heating cylinder by the rotation of the screw and has passedthrough the check valve is dissolved in the foaming gas.